1. Field of the Invention
The present invention relates to an image forming apparatus for forming a visible image by developing an electrostatic latent image formed on an image carrying member by electrophotography, electrostatic recording or the like, such as a copying machine, printer, recorded image display device or facsimile or the like.
2. Related Background Art
By a well known method according to the prior art, a dry developer is carried on the surface of a developer carrying member, this developer is carried and supplied to the vicinity of an image carrying member carrying an electrostatic latent image, and the electrostatic latent image is developed into a visible image while an alternating electric field is applied between the image carrying member and the developer carrying member. As a developing sleeve is frequently used in general as the developer carrying member, the developer carrying member will be referred to as the xe2x80x9cdeveloping sleevexe2x80x9d in the following description and, as a photosensitive drum is often used in general as the image carrying member, the image carrying member will be represented by the xe2x80x9cphotosensitive drumxe2x80x9d hereinafter.
The developing process according to the prior art, particularly in a color image forming apparatus, uses a developer consisting of two different components (carrier particles and toner particles) (two component developer) for better expression of color tints. A magnetic brush is formed on the surface of a developing sleeve with a magnet arranged inside; this magnetic brush is brought into sliding contact with or close to the photosensitive drum positioned opposite the developing sleeve with a slight developing gap in-between; and an alternating electric field is continuously applied between the developing sleeve and the photosensitive drum. This results in development by repeated shifts of toner particles back and forth between the developing sleeve and the photosensitive drum. It is known as a magnetic brush developing process.
A developing unit for this two component magnetic brush developing process has a main constitution as illustrated in FIG. 1. Referring to FIG. 1, reference numeral 21 denotes a developing sleeve; 23, a magnet roller fixed within the developing sleeve; 24 and 25, stirring screws; 18, a regulating blade arranged for forming a developer in a thin layer over the surface of the developing sleeve 21; 27, a developing vessel; and 28, a toner storage. The developing sleeve 21 is arranged close to the photosensitive drum 1 and, as illustrated in the figure, rotates in a reverse direction to the photosensitive drum 1 and so set that development can be accomplished in a state in which the developer is in contact with the photosensitive drum 1.
The two component developer is accommodated in the developing vessel 27 as a developer 19 in which toner particles and carrier particles are mixed, and the proportion of the weight of the toner particles to the combined weight of the toner particles and the carrier particles is kept constant by the dropping supply of toner in a volume matching the volume of toner consumed by the development from the toner storage 28 in which toner for replenishment is accommodated.
On the other hand, in a fixing device to be fitted to an image forming apparatus such as a copying machine or printer or the like, usually as a fixing rotator a fixing roller with a built-in heater and a press roller as a press rotator are brought into contact with each other to constitute a fixing nip, and a recording medium (transfer medium) carrying an unfixed toner image is passed through this fixing nip to have the unfixed toner image fixed on the recording medium as a permanent image by heat and pressure.
Whereas image formation using a two component developer well suited to the formation of a full color image can be accomplished with an apparatus constituted as described above, full color image forming apparatuses are now required to be further reduced in size and cost.
As a full color image forming apparatus usually requires sufficient melting and color-mixing of the toner, a soft roller made of an elastic material such as silicone rubber or fluoro-rubber or the like is often used. Also, from the viewpoints of fixing performance, image quality and toner releasing property or the like, rubber of relatively low hardness is used for the release layers of the fixing roller and the press roller, and the surfaces of these layers are further coated with a release agent such as silicone oil.
However, the above-described means involves not only the problem of requiring a complex and large fixing device but also the disadvantage of reducing the durability of the fixing roller and correspondingly increasing the cost because the coating with oil invites stripping of the elastic layer constituting the fixing roller.
To solve these problems, fluorine resin such as tetrafluoroethylene perfluoroalkylvinyl ether (PFA) or polytetrafluoroethylene (PTFE), which can be expected to be able by itself to release the toner, may be used for the release layer of the fixing roller.
This constitution according to the prior art, however, involves the following problems.
Fluorine resin, typical examples of which are cited above, is usually hard and inflexible, and is unable, especially in fixing a full color image, to follow the surface unevenness of the unfixed toner image, often destroying the toner image. This could lead to deteriorated reproducibility of dots or luster unevenness in areas of fine area. On account of this problem, an elastic layer consisting of fluoro-rubber or silicone rubber is often provided between the core metal and the release layer where the release layer of the fixing roller is made of fluorine resin.
Moreover, fluorine resin, typically PFA, has a general material disadvantage of being less resistant to abrasion and mechanical damage. On the other hand, stirring within the developing unit causes friction between the toner and the carriers or between the carriers to scrape off the unevenness of the carrier surface. Fine powder of the carriers resulting from this scraping, together with the toner, forms an unfixed image, which sticks to the surface of the fixing roller.
As a result, the fine powder stuck to the fixing roller finds its way between the components of the cleaning mechanism including a cleaning roller and a cleaning pad, sticks to the surface of the fixing roller, accumulates on a thermistor for sensing the surface temperature of the fixing roller, and damages the release layer of the fixing roller. This may invite a flaw in the contact (powder accumulating) part of the thermistor or a defect in the image known as an offset. Especially where the fixing roller consists of an elastic layer and fluorine resin, as the thermistor part cuts into the fixing roller, accumulation of the fine powder on the thermistor may accelerate damaging of the fluorine resin.
The present invention is achieved to provide a compact and low cost image forming apparatus and image forming method capable of restraining the occurrence of flaws and image defects, including offsets, even where PFA or some other fluorine resin is used for the release layers of the fixing roller and the press roller.
In order to resolve the above described problems, according to the invention, there is provided an image forming apparatus having image forming means for forming an electrostatic latent image corresponding to an image information signal on an image carrying member, forming an unfixed image by developing the electrostatic latent image using a two component developer consisting of toner particles and carrier particles and transferring the unfixed image to a transfer medium; and fixing means, provided with at least a fixing roller and a press roller, for nipping and deliverying the transfer medium with these rollers to fix the unfixed image, wherein the carrier particles contain at least binder resin and magnetic metal oxide and have shape factors SF-1 of 100 to 150 and SF-2 of 100 to 150, and the fixing roller has an elastic layer and a release layer consisting of fluorine resin, with its Asker-C hardness ranging between 60xc2x0 and 85xc2x0 at the time of application of a load of 1 kg.
According to the invention, there is also provided an image forming apparatus having image forming means for forming an electrostatic latent image corresponding to an image information signal on an image carrying member, forming an unfixed image by developing the electrostatic latent image using a two component developer consisting of toner particles and carrier particles and transferring the unfixed image to a transfer medium; and fixing means for nipping and deliverying the transfer medium with a fixing roller and a press roller to fix the unfixed image, wherein the carrier particles are a magnetic resin carrier containing binder resin, magnetic metal oxide and non-magnetic metal oxide produced by polymerization and the surface layer material of at least either the fixing roller or the press roller contains silicone rubber or fluoro-rubber.